The use of transparent glazing materials utilizing polycarbonate resin as a structural component for windows, windshields, and the like are well known. While these polycarbonate resins are easily fabricated into the desired shape and have excellent physical and chemical properties, such as being less dense than glass and having more breakage resistance than glass, their abrasion and chemical solvent resistance is relatively low.
In order to overcome this relatively low abrasion resistance and to otherwise improve the surface characteristics of the polycarbonate, various coatings have been applied onto the polycarbonate substrate. U.S. Pat. No. 3,582,398 describes a fabricated polycarbonate part having improved optical properties consisting of a polycarbonate substrate having a transparent coating thereon consisting of a thermoplastic polymethyl methacrylate. U.S. Pat. No. 4,061,652 describes a coating for polycarbonate resins comprised of (i) an acrylic resin which is a mixture of olefinically unsaturated organic monomers in combination with an acrylic polymer, and (ii) certain urethanes of hydroxybenzotriazoles and hydroxybenzophenones in combination with certain catalysts. U.S. Pat. Nos. 3,451,838, 3,986,997 and 4,027,073 disclose organopolysiloxane coating compositions and techniques for the application of these organopolysiloxane coatings onto polycarbonate surfaces. While these coatings have many desirable properties, e.g., they are hard, abrasion resistant, and chemical solvent resistant, these organopolysiloxane coatings do not in all instances possess the requisite degree of uniform adherence to and durability on these polycarbonate surfaces. U.S. Pat. No. 3,707,397 describes a process for providing a hard coating on, inter alia, polycarbonate articles, said process including priming the polycarbonate surface with an adhesion promoting thermosettable acrylic and applying an organopolysiloxane onto the primed surface. This reference further teaches that the thickness of the thermosettable acrylic polymer primer layer varies between 0.01 mil up to as much as 0.5 mil or even more.
However, organopolysiloxane coated polycarbonate articles which contain a thermoset acrylic polymer as primer have heretofore produced erratic results, i.e., in some cases, the polysiloxane top coat adhered and weathered well while in other instances the top coat failed to adhere satisfactorily or failed to weather well.
It has now been surprisingly discovered that if the primer layer is of a thickness of from about 0.015 to about 0.15 mil, then both the adhesion and weatherability of the organopolysiloxane top coat are good. If the primer layer is less than about 0.015 mil thick, the weatherability of the top coat is poor. If the primer layer is thicker than about 0.15 mil, then the adhesion of the top coat to the polycarbonate is poor.
The dependence of the performance of the organopolysiloxane top coat upon the thickness of the primer layer is surprising and has not heretofore been known or recognized by the prior art.